1. Field of the Invention
The present invention relates to a molded article comprising a thermoplastic resin containing carbon fibers. More particularly, the present invention relates to a molded article comprising a thermoplastic resin and carbon fibers, which is excellent in appearance, mechanical strength and electric properties.
2. Description of Related Art
Due to the rapid spread of electrical appliances, there have been increasing demands for shielding electromagnetic radiation by imparting conductivity to the housings of personal computers, cellular phones, parts of automobile and the like, for the purpose of preventing the ill effects associated with the electromagnetic fields caused by the electrical appliances. With regard to integrated circuits (IC), an IC tray, which contains an IC, are conventionally prepared from thermoplastic resins having good moldability. In order to protect the IC from the harmful effects of static electricity, it is necessary to make the IC trays of a conductive thermoplastic resin which imparts an antistatic function thereto. Furthermore, the metal parts of reels for magnetic tapes and paper feed rollers for copiers are just now being replaced by moldable thermoplastic resins. Accordingly, there have been increasing demands for imparting conductivity to the cassette reels and paper feed rollers to prevent static electricity.
Attempts to impart conductivity to thermoplastic articles include using conductive polymer resins. However, conductive polymer resins are very expensive and are not suitable for practical use. Accordingly, conventional thermoplastic resins available on the market have been employed to impart conductivity to molded articles. Two typical methods have been used to impart conductivity to the common thermoplastic article. The first comprises forming a conductive coat on the surface of a thermoplastic molded article by plating or vapor deposition. The second comprises mixing conductive materials such as a metal powder, metal fiber, carbon black or a carbon fiber with the thermoplastic resin and molding the composite resin into an article.
Among the above two methods, the first is inferior in productivity since it comprises two steps, i.e., a molding step and a conductive coat forming step. On the other hand, the second is more productive since it comprises just one step, i.e., a molding step.
Regarding the method of blending conductive materials with a thermoplastic resin, the conductive materials are generally in the form of powder or a fiber. When the conductive materials and resin are blended at the same ratio, an article containing fibrous materials shows more excellent conductivity than one containing powdery materials. In view of the fact that conductivity across the entire length of the article requires a continuous conductive medium between the electrical contacts, the electric charge must be able to traverse the space between the individual conductive fibers/particles dispersed in the thermoplastic resin, therefore, the materials in the form of a fiber are more likely to be in "electrical contact" with each other than those in the form of powder. Therefore, it naturally follows that longer fibers provide better conductivity than shorter fibers. As explained above, carbon and metal fibers are known to effect good conductivity. Of these, a carbon fiber having excellent affinity to the resin, especially a long carbon fiber, is most preferable since it effects a high conductive function.
Conventional methods for imparting conductivity to molded articles using long carbon fiber are classified into two types.
The first method comprises the steps of chopping a roving of the carbon fiber, melt mixing the chopped carbon fiber with a thermoplastic resin using an extruder to obtain pellets containing the carbon fibers, and then molding the pellets into an article. However, the obtained article usually does not have the requisite properties, especially good electromagnetic interference shielding, because a uniform distribution and adequate length of carbon fiber in the article cannot be obtained.
In order to avoid the defects associated with the above method, a second method has been proposed. This second method comprises the steps of coating a roving of the carbon fiber with a melted resin using either an extruder or a bath while aligning the roving through tension, cutting the roving coated with the resin into pellets, and molding the resultant pellets to obtain an article. For example, according to this method, a molded article has been proposed in which the carbon fiber retains a relatively long length (Japanese Patent Publication Examined No. 5-26828). Particularly, the molded article comprises a thermoplastic resin and 5 to 35 parts by weight, based on 100 parts of the resin, of carbon fiber having a length of 1.5 to 15 mm (the 5 to 35 parts of the carbon fiber correspond to 4.8 to 26% by weight based on the total amount of the thermoplastic resin and the carbon fiber). Upon comparison of the article obtained according to the first method, the strength and electromagnetic interference (EMI) shield of this article are improved. However, these improvements are not sufficient enough to satisfy the current demand for even higher strength and shielding properties. In addition, these articles containing carbon fibers which are prepared with either longer or in a higher concentration, in an attempt to achieve excellent strength and EMI shielding, have such a poor appearance to apply to practical uses.